The arthropods are the only phylum covered in my course that is split over multiple sessions, and with good reason. Arthropods are a majority of the species known to science. Parse that carefully: not a majority of invertebrate species. Not a majority of animal species. A majority of species, period.
That massive diversity means that arthropods are also impressively different from one another across their various groups, and covering that diversity requires an extended fraction of the lecture course’s time and two entire lab sessions. Which groups are covered in which sessions changes from year to year, so for this series I’m doing the three “classical” arthropod subphyla–crustaceans, chelicerates, and “atelocerates”–each as their own post. Today’s topic is crustaceans.
The crustaceans have not always been recognized for their distinctiveness. For a long time, they were bunched in with insects, centipedes, and millipedes as the “Mandibulata,” a group with mouthparts similar to each other’s and highly dissimilar from those of the chelicerates. Recently, crustaceans have been regarded as weird enough to warrant a third category of their own, creating the crustacean/atelocerate distinction. Amusingly, a recent high-profile genetic phylogeny shows that insects may well be highly derived crustaceans, reestablishing a revised Mandibulata and making this one clade of arthropods the dominant biota of land and sea.
Figure 1 from Regier, J. C. J. W. Shultz, A. Zwick, A. Hussey, B. Ball, R. Wetzer, J. W. Martin and C. W. Cunningham. 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature 463:1079-1083. Borrowed from Why Evolution is True.
Taxonomic weirdness aside, crustaceans in the colloquial sense are a fairly visually distinctive group, and one which has a variety of interesting and poorly understood representatives.
One thing I try to impress on my students early is that, while crabs, lobsters, and large decapod shrimp are the first things that most people think of when they hear the term “crustacean,” the majority of non-insect crustaceans are in fact small, planktonic species. These animals are probably the most important component of marine and freshwater zooplankton in all of the world’s oceans, and a lot of them are impossibly weird-looking. My personal favorite, other than the bizarre larval stages of various large benthic species, is Phronima.
Phronima, also known as the pram bug, hollows out the body of a salp, a peculiar filter-feeding chordate we’ll see again in a later installment, and lives inside it. Like many planktonic animals, both Phronima and the salp are mostly transparent, enabling them to avoid detection by their predators. Phronima‘s anatomy is adapted mostly for holding itself inside these soft shells and for moving about by pushing water out through the dead salp’s siphons. So, those imposing claws are mostly not for combat. Also, the animal is a handful of centimeters long, and its salp home is about the size of a large kiwifruit.
Phronima‘s transparency raises another interesting issue. Most superhero and science-fiction stories involving invisibility ignore or handwave away one important caveat that has kept invisibility away from human applications until now. Being invisible means that light passes through or around one’s body without interacting with it. But, interacting with light is how visual systems work. A totally invisible creature is also, inevitably, blind.
Many crustaceans, including Phronima, solve this problem by being totally transparent…except for their retinas. The most striking example of this is the giant ostracods, or seed shrimp.
Ostracods take this paradigm and run with it, and from a distance their best-camouflaged representatives look like giant disembodied retinas. Up close, one can see the carapace encompassing most of their limbs and the external feathery antennae with which it swims.
Camouflage via transparency is not in the cards for these two animals. The robber crab, below, is the largest land arthropod.
Robber crabs are a species of land hermit crab whose fleshy abdomen re-hardens during adulthood. Their natural diet includes coconuts, which they can tear open with their impressive claws. I tell my students to count the crab’s legs when the video permits. As members of the Anomura clade, robber crabs’ last set of walking legs is reduced and tucked under the abdomen. Hermit crabs use these legs to hold themselves inside their borrowed shells; other anomurans have other uses for them.
Similarly breathtaking is the giant Japanese spider crab, the largest living arthropod in any environment.
The largest specimens of these crabs have claw spans of 3.8 meters and carapaces large enough to fit over a person’s head like a helmet. They live in deep ocean waters that are extremely stable and very cold. Their hunting strategy is to spread their claws wide and wait for something to accidentally swim into them. Even a large animal in this impoverished environment, if it is a slow-moving ectotherm like this one, can get enough to eat this way.
A rather different environmental challenge sets up the tadpole shrimp’s ecology.
Also known as Triops or dinosaur shrimp, the tadpole shrimp is a relatively common item in toy stores, sold alongside the better known Artemia / brine shrimp / sea monkeys. Triops is a freshwater species that lives in ephemeral ponds in deserts. When water is available, their eggs hatch and the shrimp feed, grow, mate, and cannibalize each other as rapidly as they can before the ponds dry out again. The eggs can remain in a state of suspended animation for years at a time as they wait for the next rainfall. If the ponds last a reasonably long time, the adults can reach 10 centimeters in size and become rather striking animals. Their resemblance to horseshoe crabs is, of course, convergent.
Crustaceans have another sex-related gem among them.
Barnacles, crustaceans that people always seem to think are mollusks or plants or something, are unique in a way other than being, essentially, shrimp glued to rocks by their heads. Barnacles also have the largest known penises in relation to their body size, as demonstrated in this video. A barnacle’s penis can be many times longer than the barnacle itself, surpassing even the famous duck in relative size. This enables the barnacle to mate with its neighbors while being sessile and without recourse to the “broadcast spawning” method used by many other sessile animals.
Another breeding-related spectacle is the great crab migration of Christmas Island.
The tropics have many species of land crabs, but all lineages of land crabs are recent evolutionary events. Most of them, for example, still go through the same sequence of larval development as their marine kin, and can only breathe air as adults. So, every year, the red land crabs of Christmas Island (an Indian Ocean territory of Australia) migrate out of the inland rainforest toward the coast. At the coast, they cling to rocks and shake their eggs into the surf, trying very hard not to fall. Land crabs, you see, can neither swim nor breathe water as adults, and will drown if pulled out to sea.
A few months later, their babies have completed their various larval stages and return to shore by the millions. Once on land, they clamber toward their inland forest home, a massive tide of tiny red crabs that the islanders deal with via broom and dustpan. Those that survive the migration eventually migrate the other way as full-size crabs, to repeat the cycle.
Last but not least, I show my students the peacock mantis shrimp, Odontodactylus scyllarus.
Mantis shrimp are part of a lineage (the Stomatopoda) whose close relatives are all extinct. Unlike ordinary shrimp, their primary attack appendage is a “raptorial claw” that is lashed out at extremely high speed. Some shrimp are “spearers,” whose pointed raptorial claws are aimed at fish. O. scyllarus is a “smasher,” specializing in breaking mollusk and crustacean shells. For either variety, a mantis shrimp’s raptorial claw moves so quickly that it generates sonoluminescence and thousand-kelvin temperatures via a tiny plasma explosion with each strike. Both, unfortunately, are too short-lived to be of biological significance, but the claws are, on a large shrimp, strong enough to break glass and to earn them the name “thumbsplitters” in their native Fiji.
Mantis shrimp are also notable for being able to see circularly polarized light, which may contribute to their being on the short list of invertebrates that are confirmed as being capable of recognizing specific individuals of their own species. Either way, they are far more animated and aggressive than most crustaceans, as this particular shrimp’s handy defeat of various smaller crustaceans succinctly demonstrates. The agility of the coral banded shrimp and size of the arrow crab mean little to it, and only the large hermit crab gives it enough pause to even once use its armored telson to protect its vulnerable eyes. Watching it reel in the arrow crab by one of its legs, and run off with the autotomized claw of the coral banded shrimp, one wonders whether they aren’t also smarter than the average crustacean.
It’s no wonder, really, that The Oatmeal devoted an entire comic to the mantis shrimp’s splendor, or that the Lurker’s Guide to Stomatopods exists.